Having worked with really big transmitters; Telefunken 100 kW ISB and
Thomson-CSF 500 kW carrier AM,
tune-up works in very similar ways for all practical purposes.
First the low-power drive stages are peaked up for maximum drive voltage to the
penultimate
driver grid. After this, the procedures differ somewhat between linear
amplifiers for ISB and AM transmitters that are
Class C in the drivers and PA stages.
A servo-tuned ISB transmitter uses the phase relations between the grid and
plate circuits to establish resonance,
and then is the plate circuits and their coupling circuits adjusted to present
the proper load-lines to the tubes.
Finally, the drive level is adjusted so the proper output power and linearity
is reached.
In order for this to work properly, the tuned circuits need to be pre-set quite
closely to their final positions.
This was mostly accomplished by measuring the input frequency and use this
information to access tables of stored
tuning information, either established after the last successful tuning
operation, or at the design stage of the amplifier.
"Before computers" this was done by dividing the tuning range of the amplifier
into many sub-ranges and then use the measured frequency for choosing one of
the ranges with its preset information. After this, "Auto-tune" mechanisms with
servo motors were first brought to their initial positions, and after this
phase and load-line discriminators took over control.
Class-C transmitters used grid and plate currents to establish resonance
conditions, and were tuned progressively from the drivers to the final stages
at reduced power. As the final step, the drive was increased to reach the
proper carrier output level before high-level modulation could be applied.
The Thomson-CSF transmitters did not servo-tune in the proper sense of the
word, but used stored settings for each frequency. Instead, the servo-motors
brought the tuning elements to their pre-set positions without any room for
adjustments. This worked well because the tuning settings become quite broad
when operating into a broad-band antenna, and there are no critical linearity
specifications for a Class-C AM transmitter.
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från : amps@contesting.com
Datum : 2022-09-13 - 00:06 ()
Till : amps@contesting.com
Ämne : Re: [Amps] 10 kw CCS on 6M
One of the things I see that is different, as I have watched videos of the
bigger transmitters firing up, is that they can bring the driver stages online,
and have them drawing plate current, tune them, and then engage the final.
Obviously, there is not tx/rx relay for a plain transmitter. So, where does
that drive power go while the driver(s) are being tuned, and the final is on
standby? There are plenty of such videos out there that are exciting to watch!
FM of course, does not have any 'swing", so you crank the carrier where you
want it to be, and it just sits there. AM on other hand of course, is set for
about 25% of max carrier, and them modulation takes it up and down from there.
Based upon that description, when WLW was running a full 500KW, I'd assume the
engineer would set the carrier around 125KW, and then 100% modulation would
peak at 500KW.
Any way you slice it, that is a bunch of RF up the feedline!
Don N8ECH
In our case, we engage the tx as a unit, and then tune for appropriate
readings, and/or maximum smoke, lol.
On Monday, September 12, 2022, 03:19:34 PM EDT, Peter Voelpel
<dj7ww@t-online.de> wrote:
Yes, no difference, just bigger and more powerful.
73
Peter, DJ7WW
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Kim Elmore
Sent: Sonntag, 11. September 2022 03:36
Don Fox raises a question I've wondered about for years: how are these very
high power amplifiers initially placed in service? Today, we have VNAs we
can put in the palm of our hand, how is it done now and how was it done when
there was nothing but vacuum tubes? How adjustable are the PA output
circuits? It can't possibly be done the way we tune our puny 1.5 kW amps.
Kim N5OP
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